Xerography is a process wherein toner is selectively transferred onto a substrate and then fused. Initially, the toner is in a sump from which it is developed onto a photoreceptor. Often, the photoreceptor is a drum with a photoconductive coating. Using principals of static electricity, the surface of the photoreceptor receives an electrical charge. A light beam scanned over the surface of the photoreceptor can selectively discharge the photoreceptor surface. The toner is then developed onto the photoreceptor where it adheres, due to electrostatic attraction, to those regions of the photoreceptor that have been discharged. The photoreceptor is thereby coated with patterned toner.
A substrate, such as paper, can also receive an electrical charge. The substrate can be given a larger electrical charge than the photoreceptor so that the patterned toner is transferred to the substrate when the photoreceptor contacts it. The substrate is then heated and pressed so that the patterned toner fuses to the substrate surface. Those skilled in the arts of printing, photocopying, and xerography know the details of the xerographic process, the components used in the xerographic process, and the variations in the process details and components that occur in different embodiments of xerography.
When a drum photoreceptor is used, the drum rotates past the charging device, the exposure device, and the development device. The direction that the drum rotates is the process direction.
A light beam scanning across the photoreceptor can discharge the electric charge on the surface of the photoreceptor. Tracing a light beam across the photoreceptor creates a line, called a scan line. Turning the light beam on and off during a scan can cause selective discharging along the scan line. Making many scan lines as the photoreceptor moves with respect to the scanning beam can cause selective discharging within an area.
For example, a laser printer can have a light beam that sweeps across the surface of the photoreceptor 6000 times per second, and a photoreceptor that moves, such as a drum rotating, at ten inches per second. The printer produces 600 scan lines per inch along the process direction because the photoreceptor is moving. If the photoreceptor is 10 inches wide, modulating the light beam so that it can change 6000 times as it sweeps the photoreceptor in the less than 1/6000 of a second it takes to move across the photoreceptor results in a 600 dot per inch resolution perpendicular to the process direction.
FIG. 4, labeled as prior art, illustrates one way to obtain a modulated light beam. A laser 401 produces a light beam 403 that passes through a modulator 402 to produce a modulated light beam 107. Those skilled in the art of optoelectronics know of many devices that can modulate light beams or laser beams.
FIG. 5, labeled as prior art, illustrates another way to obtain a modulated light beam. A laser diode 501 can produce a modulated light beam 107 directly without the need for a separate modulator such as the modulator 402 of FIG. 4.
In the example above, a laser printer produced 600 scan lines per inch at a process speed of 10 inches per second. One technique to produce a higher process speed is to sweep the laser at a higher rate across the photoreceptor. Another solution is to produce many scan lines concurrently. Producing many scan lines concurrently requires many modulated light beams.
FIG. 6, labeled as prior art, illustrates one way to obtain multiple light beams. A source light beam 601 passes into a splitter 602 that splits it into numerous light beams 403. Those skilled in the art of optics know of many devices and combinations of devices for use as a splitter 602. The numerous light beams 403 can then each be modulated individually. In some applications, it can be advantageous to modulate the source light beam 601 such that it is split into numerous modulated light beams. Using numerous laser diodes or similar subassemblies can also produce numerous modulated light beams.
FIG. 7, labeled as prior art, illustrates a motor polygon assembly (MPA) 108 causing numerous modulated light beams 107 to concurrently produce multiple scan lines 113 on a substrate 112. The MPA 108 is an optical element that has many facets arranged around a rotational axis. As the MPA spins, each facet reflects the modulated light beams 107 and causes them to scan across the substrate 112 creating scan lines 113. A new set of scan lines begins as each facet starts reflecting the modulated light beams 107. Advancing the substrate along the process direction controls the locations of the new scan lines.
As with any printed output machine, a xerographic engine can exhibit print density variation. One type of print density variation is banding perpendicular to the process path. Common sources of banding are irregularity in the facets of the MPA, vibrations of the MPA, improper rotation of the MPA, and variations in the spacing and intensity of a multiple beam MPA. The effect is that the light beams sweeping across the photoreceptor have varying intensities and spacings. The differences in the intensity and spacing cause differences in discharging of the photoreceptor surface and ultimately differences in amount of toner developed onto the photoreceptor and transferred and fused to the substrate. When the exposure subsystem is the source of the banding, the period of banding in scan lines is often a multiple of or a subharmonic of the number of facets in the polygon. For example, a MPA with 16 facets has a single facet that doesn't reflect as strongly as the others. If the MPA is reflecting a single light beam to produce a single scan line at a time, then the bad facet will less efficiently discharge the photoreceptor once every 16 lines, resulting in lighter development with the same period. If two light beams are reflected to produce two scan lines concurrently, then thirty good scan lines are followed by two bad scan lines in a repeating pattern.
A need therefore exists for systems and methods that can compensate for banding due to MPA issues. Such a goal can be accomplished by changing the intensity of the light beams on a facet-by-facet basis in a way that compensates the banding on the print.